Here's the AnswerThanks very much to all of you who sent in suggestions.
Neil Sorens sent in links to two very specific threads (one and two) that discussed this exact problem. There were multiple suggestions (including using Tweak UI and registry editing), but what finally worked was a suggestion to unplug all USB 2.0 devices and reboot. I did so, did a warm boot, and plugged them all in when I got to the Windows login screen. It worked.